Data transmission methods are used whenever data needs to be interchanged by radio over short distances, that is to say, by way of example, during data interchange between the base part and the mobile part of a cordless telephone. Other examples relate to the data interchange between a PC and peripheral units, between the gamepad on a gaming console and the associated fixed station etc.
For digital cordless communication systems, such as WDCT, Bluetooth, HomeRF, various licence-free frequency ranges are reserved, particularly the “ISM” (Industrial, Scientific and Medical) frequency ranges, for example, at 2.4 GHz. Data transmissions from the base station to the mobile stations is called “down-link”. The opposite case of data transmission from the mobile stations to the base station is called “up-link”. Normally, data bursts between the stations are interchanged on the basis of a timeslot method or TDMA (Time Division Multiple Access) method.
For use of the ISM frequency bands, the appropriate standardization authority, the Federal Communication Commission (FCC), has formulated rules for how data interchange is to take place. One of these rules states that wireless data transmission must take place in line with a frequency hopping method (frequency hopping spread spectrum). In addition, there is a stipulation for how many frequency changes need to be made within particular periods of time. The available frequency band is split into various sub-bands or frequency channels. During connection, the users' transceivers hop from one channel to the other on the basis of a pseudo-random pattern.
The Bluetooth method involves the use of a frequency hopping method and a time division duplex method. The channel is split into timeslots having a length of 625 μs each, with each individual time slot being assigned a dedicated hopping frequency. Each timeslot can transmit one packet. Consecutive timeslots are used alternately for transmitting and receiving (time division duplex, TDD).
The transmission quality of the data transmission systems described is significantly impaired by virtue of the ISM frequency band being open to anybody, which means that the radio systems operating in this frequency range have to be prepared for unforeseeable sources of interference, such as WLAN transmission devices, microwave ovens, baby monitors, garage door openers and other cordless telephones. These sources of interference usually have the property that they take up a portion of the ISM frequency band statically (e.g., WLAN). For this reason, not all channels in the frequency band have the same transmission quality. A method which uses a fixed hopping sequence therefore interferes with other systems and also has its transmission impaired on the channels with interference. It is therefore fundamentally appropriate to use an adaptive scheme which endeavours to look for a portion of the available frequency spectrum which is unused by sources of interference.
To date, transmission systems that are based on a frequency hopping method have used a stipulated hopping sequence. Errors in transmission have either been accepted or have been reduced by transmitting the data repeatedly (but not in the case of voice data). These measures show no consideration for the interfering system, whose transmission is likewise impaired by the collision. In other methods known from the prior art, channels with interference are readopted into the hopping sequence after a stipulated time, without checking whether the interference on these channels still exists. If the channels still have interference, they are then removed from the hopping sequence again. This necessitates increased communication complexity however, since both the re-adoption of the frequencies into the hopping sequence and the removal of the frequencies need to be communicated within the entire system. In addition, the interfering system is impaired during this time.